The non-ideal effects associated with channel loss in connection with broadband data communication systems such as high-definition television (HDTV) impact signal quality increasingly as the bit rate increases. In particular, non-ideal effects such as skin effect loss and dielectric loss in the channel, e.g., cable, printed circuit board trace, and the like, attenuate data more significantly at higher frequencies. Indeed, data attenuation can be represented by the following transfer function, L(f):L(f)=e−I(ks√{square root over (jf)}+kd|f|)  (1)where f is the frequency, I is the channel length, and ks and kd are the skin effect loss constant and dielectric loss constant of the channel, respectively.
One way avoiding bit errors and inter-symbol interference (ISI) that results from the interference between adjacent pulses and for receiving a high quality data signal is “equalization”. Equalization counteracts channel loss to compensate for transmission loss and to recover the distorted signal using an inverse or reciprocal frequency transfer function as the channel loss, i.e., 1/L(f). Because the exact characteristics of the channel are unknown, adaptive equalization is preferable to fixed equalization. Adaptive equalization refers to the ability of the system to adapt to find the proper compensation level for a specific channel.
An example of a conventional adaptive equalization system can be seen in FIG. 1. The equalization system 10 includes an equalizer 15 and an equalizer loop 18 that provides a control voltage 11 to the equalizer 15 to achieve a desired equalization level. More particularly, the equalizer loop 18 includes a low-pass filter (LPF) 12, a high-pass filter (HPF) 14, rectifiers 17 and 19, and an integrator 13. In operation, output from the equalizer 15 is provided to both the LPF 12 and HPF 14, which extract signal energy within the respective frequency bands. Filter outputs are rectified by the pair of rectifiers 17 and 19, and, then, integrator 13 attempts to adapt the equalizer 15 by adjusting the control voltage 11 (which is applied to a control voltage terminal of equalizer 15).
The ratio between the signal energies of the LPF 12 and the HPF 14 is preset and fixed, e.g., the ratio of high-pass-to-low-pass filter signal energy can be preset and fixed at 1:1. However, in practice, the adapted operating point is not fixed so the high-pass-to-low-pass filter signal energy, typically, is not 1:1. The high-pass-to-low-pass filter ratio is variable due to, for example, the channeling medium, the transmitted data, process, supply voltage, temperature, and the like. Accordingly, the control signal 11 of the adaptive equalizer 15 may be imperfect, resulting in a correspondingly incorrect or non-ideal equalizer gain setting.
In either instance, over-equalizing or under-equalizing an attenuated input signal causes jitter. Consequently, it would be desirable to provide a self-calibrating adaptive equalization system to improve jitter performance.